The present invention relates to a method and apparatus for recording keyboard music for re-creation on a similar keyboard instrument by actuation of the keys and, more particularly, to the detection, encoding, recording and reproduction of expression effects on electronic keyboard instruments. Expression control has been provided in a number of ways in the prior art. As disclosed in Vincent U.S. Pat. No. 3,905,267, transducers, such as microphones, accelerometers or magnetic pickups produce voltages which are proportional to the intensity with which the keys are struck. The information is then digitized in an analog to digital converter and combined with the keyboard switch actuation signals. These systems do not take into account the delay between the key switch actuation and the actual production of the musical note involved nor do they take into account the mechanical differences between the production of notes in the treble range as compared to the production of notes in the bass range. Finally, the digital multiplex word format placed the expression bits for both bass expression and treble expression in one position in each frame after the key switch bit data.
In accordance with the present invention, the expression effect utilizes a single simple microphone to detect the variations in the intensity of the acoustic waveform music being recorded and senses the changes in power by performing a digital integration of the waveform produced by the microphone. The output of the microphone, however, is first preamplified and then applied to a low pass filter for the bass notes and a high pass filter for the treble notes. When the music waveform for either the bass or treble section of the keyboard is greater than the threshold, a clocking signal is allowed to advance a binary 5-bit counter unit with a 31-count range. The counting system is adjusted by presetting the d.c. bias level so that the maximum volume required from the piano in either the bass or treble section produces the maximum count (31) from the counter. Thus, the longer the music waveform is above the threshold, the higher the expression stored. The integrating system can be adjusted to compensate for the higher frequency and thus the lower counts of the treble notes by setting the basic threshold of the treble comparator slightly lower than that of the bass. The system also compensates for the playing of more than one note by counting the number of notes played and automatically raising the threshold when the multiple notes are sounded.
Finally, the key data from the key switch multiplexer is applied to a pair of serially connected 128-bit shift registers. The output of the first shift register is supplied to the second shift register and to an OR gate along with the output of the second shift register so that every key switch closure or actuation extends over two time frames and is, in effect, a note stretcher. This note stretching removes the very sharp and mechanical sound from short notes.